Numerous techniques are known for introducing DNA into eukaryotic cells, often by means of vectors, and often in the form of nucleic acid encapsidated by a (frequently virus-like) proteinaceous coat.
Many such techniques have been proposed, and some used, for a wide range of clinical as well as experimental applications.
The deliberate introduction of DNA encoding a desired gene, under conditions where the gene may be expressed within the cell and leads to the production of RNA and/or protein, can be desirable in order to provoke any of a wide range of useful biological responses.
It has recently been reported that injection even of free (naked) plasmid DNA directly into body tissues such as skeletal muscle or skin can lead to protein expression and to the induction of cytotoxic T lymphocytes (CTLs) and antibodies against the encoded protein antigens contained in the plasmid (Ulmer et al, Science, 259 (1993) 1745-1749; Wang et al, Proc Nat Acad Sci U.S. 90 (1993), 4157-4160; Raz et al, Proc Nat Acad Sci U.S. 91 (1994), 9519-9523). Nevertheless, the efficiency of nucleic acid introduction by virus-like vectors can often be higher than the effect of naked DNA and there are continuing efforts to identify and make improved virus-like vectors.
Among known virus vectors are recombinant viruses, of which examples have been disclosed based on several virus classes including poxviruses, herpesviruses, adenoviruses, and retroviruses. Such recombinants can carry heterologous genes under the control of promoters able to cause their expression in vector-infected host cells. Recombinant viruses of the vaccinia and other types are mentioned and cited for example in a review by Mackett, Smith and Moss (J Virol 49(3) (1994) 857-864).
By way of further example, specifications WO 88/00971 (CSIRO. Australian National University: Ramshaw et al), and WO 94/16716 (Virogenetics Corp: Paoletti et al) describe recombinant vaccinia viruses carrying genes for a variety of cytokines, e.g. GMCSF, and reported effects include potentiation of the immune response to recombinant viruses carrying cytokine genes.
Examples of further known recombinant virus vectors, for example of the vaccina virus and herpes virus types, are disclosed in specifications WO 92/16636 (Cantab Pharmaceuticals: Boursnell et al) and WO 92/05263 (Immunology Ltd: Inglis et al) and WO 94/21807 (Cantab Pharmaceuticals: Inglis et al), where recombinant viruses are disclosed that carry genes for heterologous antigens intended to evoke an immune response in a treated subject.
The prior art also includes amplicons based on herpes viruses such as herpes simplex virus, as described for example by Vlazny and Frenkel, in Proc Nat Acad Sci U.S. 78 (1981) 742-746; and by R. R. Spaete and N. Frenkel, Cell 30 (1982) 295-304. Such amplicons are in effect DNA which is highly defective in respect of viral gene functions, but contains at least a copy of a HSV origin of replication (ORIs) and a packaging signal sequence (localised in the repeated `a` sequence of the HSV genome). When introduced into suitable cells together with a helper HSV virus, the amplicons can propagate in dependence upon propagation in the same cells of the helper herpes viruses, which provide trans replication and packaging functions (including structural components of the virions in which progeny amplicons are packaged) not encoded by the amplicons themselves.
Specification WO 94/04695 (Rockefeller University; M. G. Kaplitt) relates to a herpesvirus defective vector carrying a foreign gene `driven` by a promoter endogenous to a host that is to be infected by the vector. The prospective uses are stated to include gene therapy e.g. in connection with adult mammalian brain. In related publications, Kaplitt et al, in Proc Nat Acad Sci U.S. 91(19) (1994) 8979-8983 (and an earlier article) discuss the use of a defective herpes simplex virus vector to express a foreign gene in the adult rat brain. They describe a herpes simplex virus amplicon containing a foreign gene (bacterial lacZ) downstream of a 2.7 kb fragment of rat preproenkephalin promoter.
U.S. Pat. No. 4,996,152 (J. K. Carter et al; U.S. Sec of Agriculture) describes DNA fragments (seeds) having the characteristics of amplicons from Marek's disease virus, of which concatemers of the seeds and associated genes were described as having potential as vaccines or delivery vectors when cotransfected and replicated in the presence of helper viruses, and for use for inserting genes into the helper viruses.
A. I. Geller et al, in Proc Nat Acad Sci (1990) (Nov) 87:8950-8954 describe "An efficient deletion mutant packaging system for defective herpes simplex virus vectors; Potential applications to human gene therapy and neuronal physiology", based on use of a defective HSV1 vector system to introduction E. coli beta-galactosidase into in cells from adult rat brain, propagated in association with a helper virus deleted in respect of gene IE3, on a complementing cell line.
"Gene therapy" includes the clinical aim of replacing the function of a gene which is missing or defective, and attempts are known to treat cystic fibrosis or adenosine deaminase (ADA) deficiency by introducing into patient cells functional copies of the genes defective in these diseases (i.e. genes encoding CFTR and ADA respectively).
Another known proposal for a form of gene therapy comprises introducing a nucleic acid sequence to up-regulate or down-regulate expression of a target gene in the host cell, either by means of a protein encoded by the introduced nucleic acid sequence or by means of an anti-sense relation between RNA encoded by the introduced nucleic acid and a target nucleic acid molecule corresponding to an endogenous gene product.
Traditionally the aim of vaccination was to protect against infectious diseases, but this aim has now broadened to include other kinds of immune stimulation, for example to include treatment of tumours and some other immune diseases, and to include therapy as well as prophylaxis (prevention).
It remains desirable to provide further forms and preparations of vectors that show useful combinations of desirable properties in connection with gene delivery e.g. for purposes such as gene therapy and vaccination.